1. Field of the Invention
The present invention relates to a focus detection apparatus and a control method thereof.
2. Description of the Related Art
Conventionally, a phase difference detection method is generally known well as the automatic focus detection method of a camera. In the phase difference detection method, the images of light beams from an object, which have passed through different exit pupil regions of an imaging lens, are formed on a pair of line sensors provided on an AF sensor. The relative position (phase difference) between a pair of object images obtained by photo-electric conversion of the pair of line sensors is calculated, thereby detecting the defocus amount of the imaging lens (AF calculation).
In such a focus detection apparatus, the number of photo-electric conversion units for acquiring focus detection images recently tends to increase to attain a multipoint configuration or a high resolution in detecting the defocus amount with respect to a plurality of objects. On the other hand, along with the increase in the number of photo-electric conversion units, it is becoming very difficult to completely eliminate defects in the manufacture.
There are known following literatures in association with the automatic focus detection technique using the phase difference detection method. For example, Japanese Patent Laid-Open No. 2003-222786 discloses an AF sensor according to related art which controls accumulation based on the maximum value signal of line sensor signals. In addition, Japanese Patent Laid-Open No. 2001-177756 discloses a technique of obtaining signals for focus detection while excluding the signals of defective photo-electric conversion units based on the information of defective pixels checked in the manufacturing step in advance.
In the technique disclosed in Japanese Patent Laid-Open No. 2003-222786, however, if a photo-electric conversion pixel of the line sensor is defective, electric charge accumulation ends without obtaining a sufficient object image signal.
This will be described below in more detail. One of object images formed on a pair of line sensors is defined as an image A, and the other as an image B. FIGS. 18A and 18B show an example of signals obtained from the object images when no defective pixel exists. When the output signal reaches the maximum value determination voltage, accumulation termination determination is performed, and the accumulation period ends. On the other hand, FIGS. 19A to 19C show an example of waveforms in a dark state when a defective pixel exists. The increase in the signal caused by the dark current of the defective pixel is larger as compared to a normal pixel. FIG. 20 shows accumulation control performed for the same object as in FIGS. 18A and 18B using the maximum value signal when a defective pixel as shown in FIGS. 19A to 19C exists. Since accumulation termination determination is done by the defective pixel signal that has reached the maximum value, the accumulation period ends before the signal amount to be originally obtained is accumulated. For this reason, the object signals shown in FIGS. 18A and 18B change to those in FIGS. 9A and 9B. As a result, no sufficient signal amount can be obtained, the S/N ratio of the signal lowers, and the accuracy of focus detection calculation lowers.
As disclosed in Japanese Patent Laid-Open No. 2001-177756 described above, accumulation control may be done while excluding the signals of defective photo-electric conversion units based on the information of defective pixels checked in the manufacturing step in advance. However, this method has no effect on unknown defects that have occurred after the check in the manufacturing step.